Need Bandwidth Monitoring Tool for Linux

We have switched cloud linux recently and Centos was running on all the server, the reason for modified the OS for limit the usage of users, yet we faced a bandwidth spike on our servers from some of IPs or using scripts from 777 permission folder. This is a web hosting company we notified bandwidth usage from data center techs.

Is there any tool available to notify bandwidth usage through email if crossed certain limit for Linux?

Thanks for your reply, I am not familiar with Nagios and CACT but have installed on the servers so can you let me know how it intimates us if any bandwidth spike. Do we have to monitor via tool or will it intimate via email? It would be grate if you give me step by step instruction how set it up to avoid bandwidth spike.